The following excerpt is from The Winter Market Gardener (New Society Publishers, October 2023) by Jean-Martin Fortier and Catherine Sylvestre.
In winter, plant growth slows, water uptake tapers off, and the work related to crop irrigation winds down. That’s the good news. The bad news is that irrigation in very cold weather is a real challenge in unheated greenhouses. Water freezes in conduits, which can lead to significant damage (e.g., valves bursting or irrigation lines freezing). To successfully supply water to unheated shelters, the easiest solution is to install an antifreeze hydrant inside the greenhouse (also known as a frost-free hydrant or anti-siphon faucet). This faucet needs to be connected to a water line buried at a depth of 5 feet (1.5 m), lying below the frost line. When we open the valve, water rises in the pipe and we can irrigate crops despite below-freezing temperatures. When we close the valve, the water drops back down into the pipe, below the frost line. This freeze-proofing feature keeps water from sitting in the valve, which is the main cause of frost damage in an irrigation system. Installing this valve requires excavation work, but it is essential. When irrigating an unheated shelter without an antifreeze hydrant, valves must constantly be purged—and the threat of damage is always looming.
The type of irrigation system you select, either drip or sprinkler, needs to take into account the likelihood of freezing temperatures. If the shelter is unheated, drip irrigation is the best choice. Make sure to install connectors properly to prevent water from pooling and stagnating. Irrigation lines in a drip system can handle some expansion in below-freezing temperatures and are less likely to break. Easy to maintain, the drip system is a highly effective solution for many vegetables. It also has the advantage of delivering water right onto the ground, keeping the plant foliage dry. This is particularly relevant in winter, when relative humidity tends to be quite high.
For direct-seeded crops sown in multiple beds at once (still in an unheated shelter), drip is not an optimal irrigation system because it’s impossible to lay out the lines before the seeds germinate. In this case, after the seeding, sprinklers must be set up to water the beds until the crop germinates and becomes established. To avoid any frost damage, irrigation lines must be purged and/or stored in a heated place, such as a garage, after being used.
In minimally heated greenhouses, the threat of freezing temperatures is nonexistent and equipment damage is less likely. In this case, choosing between drip or sprinkler irrigation will depend instead on the crop and planting date. This is a significant advantage provided by minimally heated greenhouses.
When irrigating different crops, knowing when to water and for how long depends largely on the weather and the time of year. In general, especially in unheated shelters, no irrigation is needed in December and January, when day length is at its shortest. At this time, soil moisture retention is high, and the plants, which are in a phase of extremely slow growth, draw very little water up through their roots. When February rolls around and day length increases, it’s time to come out of hibernation and make sure the soil doesn’t get too dry. We monitor the crops and feel the soil several times a week. A moisture meter can also help determine if a bed is ready to be watered.
Another crucial factor to consider when watering winter crops is the relationship between irrigation and a plant’s frost resistance. Plants with a slight water deficit have been shown to better tolerate freezing temperatures because with less water in their cells, they are less likely to form ice crystals in below-freezing conditions. Since crystals are deadly, it’s important to consider this when establishing an irrigation strategy before a major frost event. It’s always safer to tend towards watering too little rather than too much, especially when nights drop below freezing in December and January.